The present invention is related to telephone subscriber line interface circuits (SLICs), and more particularly to a method and circuit for performing a loopback test of a SLIC that may also be used for on-hook transmission.
With reference to FIG. 1, a telephone system may include a subscriber telephone 12 connected to a central office through a telephone subscriber line interface circuit (SLIC) 14. The telephone 12 may be represented by a bell 12a, a capacitor 12b for blocking dc signals, voice circuitry 12c, and a switch hook 12d. The SLIC connects a balanced two wire transmission path (the path to and from the subscriber telephone) with an unbalanced four wire transmission path (the path to and from the distribution circuitry in the telephone central station). SLICs perform various functions, including battery feed, overvoltage protection, ringing, signaling, hybrid, and timing. SLIC operation is known and need not be considered in detail, and only portions of the SLIC 14 that may facilitate an understanding of the background of the present invention are shown in FIG. 1.
Telephone systems operate in potentially harsh environments and may have portions of the system exposed to faults, such as short circuits from accidental grounding, power lines, lightning, etc. that may damage system components. The overvoltage function of the SLIC is designed to protect the SLIC from such faults. If the SLIC does not detect the fault, however, the SLIC may attempt to drive the short and overheat, thereby damaging SLIC components. A loopback test may be conducted to determine whether certain SLIC components have been damaged. The loopback test is conducted when the telephone is not is use (it is on-hook) by switching a dummy load to the line. In FIG. 1, the dummy load is represented by resistor R.sub.dum that may be selectably connected across the line with switch 16. An ac signal is typically applied to the receive pin Rx at the four wire side of the SLIC. The ac signal appears at the two wire side of the SLIC across the dummy load and thereafter at the transmit pin Tx at the four wire side of the SLIC. The amplitude of the ac signal at the transmit pin indicates whether a component has been damaged.
Another function of a SLIC is to supervise telephone operation by detecting when a request for service is made at a telephone. The request for service is made by lifting the handset from the cradle, thereby activating the telephone's hook switch. As indicated in FIG. 1, the request for service is made by closing switch 12d. When switch 12d is closed, a dc load is applied between nodes VT and VR. The closing of switch 12d is referred to as a switch-hook and is detected by the SLIC.
Switch-hook detection (SHD) occurs when the dc current through the telephone line (the line current) exceeds a predetermined magnitude, referred to a threshold current, I.sub.SHD. As indicated in FIG. 1, the line current comprises a current I.sub.T that is proportional to tip voltage plus the amplifier gain G times I.sub.T. Typically, a voltage across a resistor R.sub.SHD in series with the telephone is measured and compared to reference voltage in a comparator 18, where R.sub.SHD and the reference voltage have been selected so that a switch-hook detection is indicated when the line current exceeds I.sub.SHD. When the measured voltage exceeds the reference voltage, the comparator 18 sends a SHD signal to the SLIC to indicate a request for service. The request for service indication is maintained until the measured voltage no longer exceeds the reference voltage. When the measured voltage no longer exceeds the reference voltage the telephone is disconnected.
As will be appreciated, the dummy load and switch 16, and the need to provide an ac signal generator for the test signal add cost, complexity, and size to the SLIC and/or telephone system. It is desirable to be able to perform the loopback test without these components, and more desirable to bring the test function on-chip, when the SLIC has portions in an integrated circuit. Further, it is desirable to use loopback test components for other purposes to further decrease the number of components needed for telephone system operation.
Accordingly, it is an object of the present invention to provide a novel method and circuit for a SLIC that obviates the problems of the prior art.
It is another object of the present invention to provide a novel method and circuit for loopback testing a SLIC that uses the switch-hook detection circuit of the SLIC.
It is yet another object of the present invention to provide a novel method and circuit for loopback testing a SLIC in which separate current sources are connected to each of tip and ring circuits from which switch-hook is detected in the SLIC and in which a loopback test uses current from the current sources.
It is still another object of the present invention to provide a novel method and circuit for loopback testing a SLIC in which a switch-hook detection indication is provided and a ground key detection indication is not provided when the loopback test is successful.
It is a further object of the present invention to provide a novel method and circuit for performing loopback testing and on-hook transmission in a SLIC in which a loopback test uses current from separate current sources in the switch-hook detection circuit of the SLIC, and in which on-hook transmission uses current from the same current sources.
It is yet a further object of the present invention to provide a novel method and circuit for on-hook transmission in a SLIC in which separate current sources in the switch-hook detection circuit of the SLIC are used for on-hook transmission and in which a switch-hook detection threshold is reset so that a switch-hook detection is not indicated by the current sources during on-hook transmission.
It is still a further object of the present invention to provide a novel circuit for loopback testing an integrated circuit SLIC in which separate current sources are connected to each of tip and ring circuits with which switch-hook is detected in the SLIC and in which the loopback testing circuit is integrated into the SLIC IC.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.